Exemplary embodiments of the invention relate to a welding tool for joining at least two workpieces at a joining region by means of friction stir welding.
In contrast to the friction welding process, the friction energy in the friction stir welding process is not generated due to the rotation of one of the two workpieces, which are to be joined, relative to the other workpiece, but rather a wear-resistant welding tool is used. This wear-resistant welding tool applies the rotational energy simultaneously to both workpieces. This process comprises the following steps.
In the first step the welding tool comprising a rotating tool pin is pressed with high force between the two workpieces to be joined, until a shoulder, which surrounds the tool pin and is a part of the welding tool, comes to rest on the surface of the workpieces.
In the second step the rotating tool pin remains for a short period of time at the insertion point, as a result of which the area between the shoulder of the welding tool and the workpieces heats up to just below the melting point of the workpieces. At the same time the material of the workpieces becomes soft and is plasticized, so that a mixing of the materials of the workpieces, which are to be joined, in the joining region is possible.
In the third step the welding tool is moved in the feed direction in such a way that the shoulder continues to be pressed with considerable force onto the workpiece surface, so that the tool pin is pressed into the joining region. During the feed motion, a pressure difference between the front portion of the welding tool and its rear side is generated due to the rotational movement of the tool pin, so that plasticized material is conveyed around the tool pin, mixes and, thus, helps to form the weld seam.
The welding tool is extracted from the joining region at the end of the weld seam.
Working on this basis, the welding tool comprises a tool pin and, arranged more or less perpendicular to the tool pin, a shoulder with a larger diameter than the tool pin itself. The shoulder serves the purpose of isolating the surrounding air from the weld seam, whereas the tool pin is responsible for the swirling of the material. Since the material is entrained by the swirling in order to form the weld seam, there is no need for additional materials. Furthermore, it is also possible to dispense with a protective gas atmosphere.
However, the drawback is that an end-of-the weld crater at the end of the weld seam is formed by the extraction of the tool pin. Furthermore, it has also been difficult to weld a welding depth that varies between two workpieces that are to be joined, because it has always been necessary to mount a new tool pin having a different length on the welding tool.
A solution to the aforementioned problems has been to use an automatically retractable tool pin, which can be moved vertically inside a tool pin holder of the welding tool. However, such a solution makes the design of the welding tool more complicated and subject to wear.
Therefore, exemplary embodiments of the present invention are directed to a simple tool design, which is less subject to wear, and to achieve a change in the length of the tool pin.
A welding tool for joining at least two workpieces at a joining region by means of friction stir welding has a tool pin, which is arranged along one axis of the welding tool, for applying frictional heat to the workpieces and a shoulder, which surrounds the tool pin and is provided for separating the joining region from the surrounding area. The shoulder is designed in such a way that it can be moved more or less parallel to the axis.
Since the shoulder is designed to be movable, it is possible to extract the tool pin, which is surrounded by the shoulder, from the shoulder at different tool traversing lengths. As a result, it is possible to achieve different welding depths using one and the same tool pin. Furthermore, with this welding tool even workpieces exhibiting a varying or different joint thickness can be easily connected to each other without interrupting the welding process.
Preferably the shoulder has an elastic area. The advantage of this feature is that then the shoulder can be pushed back parallel to the axis by simply applying force, so that the tool pin protrudes further from the shoulder. As the force decreases, the shoulder expands again, so that the tool pin enters again further into the shoulder, thus enabling in an advantageous way shorter welding depths.
It is even more highly preferred that the elastic area is formed by a spring arrangement. As a result, an elasticity of the shoulder can be achieved in a particularly easy way through the use of heat resistant materials. As an alternative, elastic materials may be provided as a cushion at a suitable location on the shoulder, in order to be able to move said shoulder parallel to the axis through the application of force.
Preferably the elastic area is formed on a shoulder end region that is to be turned toward at least one of the workpieces during normal operation. This arrangement makes it possible to achieve a more stable design of the entire shoulder, so that only the lower portion, i.e. the shoulder end region, exhibits the elastic area, thus making the shoulder movable parallel to the axis.
The tool pin advantageously has, in the direction of the axis, a projection for projecting beyond the shoulder end region. This projection can also be called the end region of the tool pin or the tip of a tool pin.
It is even more preferred that a maximum possible motion path of the shoulder corresponds in essence to the projection of the tool pin beyond the shoulder end region. As a result, it is advantageously possible to insert the tool pin completely into the shoulder, thus cleaning the tool pin by means of the movement. For example, the maximum possible motion path can range from 0 to 10 mm, in particular from 2 to 4 mm.
Furthermore, the tool pin is advantageously mounted on a tool pin holder in a manner allowing it to be removed, so that the tool pin can be removed preferably with ease from the tool pin holder and can be replaced with a new tool pin.
In a preferred embodiment the tool pin holder is mounted in such a way that it can be rotated about the axis. Hence, when the tool pin holder rotates, the tool pin itself automatically rotates along with said tool pin holder. Therefore, a rotatable mounting of the tool pin itself can be preferably dispensed with.
Advantageously the shoulder is integrally formed with the tool pin holder. In such an arrangement the shoulder can rotate simultaneously with the tool pin about the same axis at the same angular speed.
As an alternative, the shoulder may be mounted separately from the tool pin holder in such a way that it can be rotated about the axis; or the shoulder can be arranged in such a way that it is stationary. This arrangement makes it possible to achieve, for example, a different angular speed between the tool pin and the shoulder, or the shoulder may enclose the tool pin rigidly and without rotation.
Preferably the tool pin holder is designed in the shape of a cone in the longitudinal section along the axis. Then the shoulder, which already extends along the tool pin holder in the direction of the axis, can be brought closer to the tool pin at the shoulder end region, as a result of which preferably a better seal against the surrounding air can be achieved.
Therefore, it is advantageous if the shoulder is also constructed in the shape of a cone in the longitudinal section along the axis and/or is formed in essence in the shape of an L.
A welding method, which is provided for joining at least two workpieces having a welding depth, which varies along a joining region, by means of friction stir welding using a welding tool described above, includes the steps:
a) bringing the shoulder in contact with at least one workpiece;
b) applying a force, which is directed more or less parallel to the axis, to the shoulder, until the tool pin has completely penetrated a joining region of said at least two workpieces;
c) changing the force for changing the welding depth of the tool pin in the joining region while the shoulder is in continuous contact with the joining region.
With this method even those workpieces exhibiting a varying or different joint thickness can be easily connected to each other without interrupting the welding process.
Preferably the welding process is followed by a removal of the shoulder from the at least one workpiece and by a displacement of the shoulder parallel to the axis, so that the tool pin can be easily cleaned in this manner.
A workpiece is produced preferably with the welding tool described above and/or with the welding method described above.